Philanthropist T. Denny Sanford has suddenly become shy about his generosity to the Sanford-Burnham Medical Research Institute in La Jolla.

Amid fanfare, the South Dakota businessman donated at least $70 million to the institute from 2007 to 2010. He also just made a third gift which is simply being described as “a seven-figure donation to cancer research.”

That means the gift could conceivably be anywhere from $1 million to $9 million. Sanford declines to clarify things, or say why the amount of this gift is being kept private.

In a statement, Sanford said, “At this time of transition, I want to provide both financial support and a personal endorsement of Sanford-Burnham’s excellence. I have the utmost confidence in the Institute’s future and Dr. Vuori’s leadership as interim CEO.”

Sanford was referring to John Reed’s recent announcement that he’s stepping down as Sanford-Burnham’s chief executive officer to take a job in industry. Kristiina Vuori, Sanford-Burnham’s president, was given the added title of interim CEO.

The latest gift from Sanford is intended to expand the institute’s efforts to determine how individual tumors can differ genetically, an area of research that’s considered essential to the development of new drugs. Vuori specializes in that area, with a special focus on breast cancer.

“As a cancer researcher, this latest substantial gift means a great deal to me personally, as well as to Sanford-Burnham,” Vuori said in a statement.